Micromechanical displays and fabrication method

ABSTRACT

A display device, in accordance with the present invention includes a transparent substrate and an array of pixels formed on the substrate, each pixel comprises a transparent electrode and a deformable member electrically actuated between a first state and a second state, wherein in the first state a liquid including a dye is disposed in a gap between the transparent electrode and the deformable member and wherein in the second state the deformable member reduces the gap between the transparent electrode and the deformable member such that the liquid is substantially removed between the deformable layer and the transparent electrode in the area of contact. A plurality of switches are formed on the substrate for supplying control signals to the array of pixels to selectively actuate the deformable members of the pixels, wherein each switch comprises an actuating member movable between an active state and an inactive state, whereby in the active state any control signal supplied to the switch passes through the switch, and in the inactive state any control signal supplied to the switch is prevented from passing through the switch. Fabrication methods are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to micro-mechanical devices for reflectivedisplays and, more particularly, to a reflective display havingdeformable mirrors.

2. Description of the Related Art

There are significant efforts underway to develop low power, highresolution, “paper-like” displays using either liquid crystals (T. Ogawaet al., “The Trends of reflective LCDs for future electronic paper”, SID'98 Digest, p. 217) or other technologies. The liquid crystal basedapproaches generally suffer from low reflectivity and poor contrastratios. There has recently been a number of publications on using MEM(microelectromechanical) devices for display applications. Projectionsystems based on arrays of tilting mirrors have been commercialized(See, e.g., J. Sampsell, “An overview of the digital micromirror device(DMD) and its application to projection displays”, SID '93 Digest, p.1012) and projection systems proposed using a micromechanical phasegrating (See, e.g., D. Bloom “The grating light valve: revolutionizingdisplay technology”, SPIE Vol. 3013 (1997) p. 165).

Two types of MEM based direct view displays have also been proposed. Inthe first (See e.g., E. Stern, “Large-area micromechanical flat-paneldisplay”, SID 97 Digest, p. 230), an array of passively addressedbistable transparent beams are used to control the release of lighttrapped by total internal reflection. This device uses a back light, anddue to the thick optical feed structure (about 4 cm) is not be suitablefor portable displays. A second direct view display (See e.g., M. W.Miles, “A new reflective FPD technology using interferrometricmodulation”, SID 97 Digest (1997) p.71) includes the use of amicromachined deformable optical cavity whose reflected color changeswith voltage. The device includes a self-supporting deformable membrane,made of, for example aluminum, and a thin film stack, both residing on atransparent substrate. The self-supporting deformable membrane and thethin film stack act as mirrors for an optically resonant cavity. When avoltage is applied, the deformable mirror collapses and the color of thereflected light is changed. The devices are binary and have hysteresiswhich allows passive addressing. The color selection of the two statesis determined by the optical stack (which contains a conductor) and bythe rest height of the deformable mirror. The main disadvantage of sucha system is that the maximum reflectivity is limited. For a narrow colorband, the peak reflectance can be about 80%. If an 80% reflectivity isassumed for the whole Red, Green, and Blue bands, a triad pixelstructure, and an 80% aperture ratio, the maximum white reflectivitywould be about 21%. For a paper-like display, a reflectivity of 40% ormore is necessary.

A type of display, referred to as “electroscopic displays”, have beendescribed by T. S. Te Velde et al. which are bistable and have animproved reflectivity compared to the interferrometric modulationdisplays described above. (See Te Velde et al., “A family ofelectroscopic displays”, Society of Information Display 1980 technicaldigest, p.116-117 and the following U.S. Pat. Nos. 4,178,077, 4,519,676,4,729,636.) The article entitled, “A family of electroscopic displays”(hereinafter Velde), describes an electroscopic fluid display where aplate or grid which is reflective and is movable is sealed with a glassplate and filled with a nonconducting black or other colored solvent. Ifthe penetration depth of the incident light in the solvent is muchsmaller than the thickness of the cell, than when the white grid islocated near the bottom plate, the grid will not be visible and the cellwill appear black. However, when the white grid is attracted to thefront side, the white grid will be visible and the cell will appearwhite.

Two possible configurations are described in Velde, a springy capacitorand a triode. For the springy capacitor, the grid is mechanicallyfastened to the bottom plate via conductive springs and when a largeenough voltage is applied, the springs are stretched and the grid rushesto the upper electrode. This arrangement requires careful cell gapcontrol since the threshold voltage is a function of the cell gap.

In the triode configuration, the springs are made very weak so thatmechanical forces can be neglected and electrodes on the top and bottomplates are used to electrostatically control the position of thereflective plate. In U.S. Pat. No. 4,178,077, a triode configuration isdescribed where electrostatic forces are used to control a movableelectrode in an opaque liquid. A fabrication process is also describedwhich uses an underetching process where apertures in a second layerprovide access for the etchant to the first layer. This requires a timedetch to leave portions of the first layer in place to support the secondlayer. In U.S. Pat. No. 4,519,676, a triode configuration is againdescribed, but with the resilient elements below the display part toincrease the aperture ratio. A more complicated fabrication process isdescribed which again uses timed underetching. In U.S. Pat. No.4,729,636, engaging points are formed between the movable electrode andits engaging surface to improve the response time by allowing liquidflow in and out during closure and release. The triode configuration iscomplicated and requires electrical contacts for addressing to be formedon both top and bottom plates. Both the triode and springy capacitor(when fastened to the bottom plate) require precise cell gap controlsince the threshold voltage depends on the cell gap. The fabricationprocesses described require the etching step to be stopped by a certaintime or the first layer will be fully removed and the second layer willno longer be attached to the substrate.

For a high information content display, such as one for an 8.5 inch by11 inch sized display with 150 dot per inch resolution, it isadvantageous to integrate some of the addressing electronics on thedisplay itself to reduce cost and improve yield. For the display sizedescribed above, approximately 1,275 gate line and about 1,650 data lineconnections and driver chip outputs are needed. If the displaytechnology used can also provide switches, the row selection circuits(i.e., shift register) and data driver demultiplexing circuits may bemade with the display and greatly reduce the number of connections anddrivers. (See “Silicon light valve array chip for high resolutionreflective liquid crystal projection displays”, by J. L. Sanford et al.,IBM J. Res. Develop., Vol. 42 No. 3/4, May/June 1998, pp.347-358,incorporated herein by reference.)

Therefore, a need exists for a portable display having high reflectivityand a high contrast ratio. A further need exists for a display whichpermits switches to be fabricated at the same time as the displaydevice. A still further need exists for a method for fabricating thedisplay device and switches in an efficient and economical manner.

SUMMARY OF THE INVENTION

A display device, in accordance with the present invention includes atransparent substrate and an array of pixels formed on the substrate,each pixel including a transparent electrode and a deformable memberelectrically actuated between a first state and a second state, whereinin the first state a liquid including a dye is disposed in a gap betweenthe transparent electrode and the deformable member and wherein in thesecond state the deformable member contacts the transparent electrode todefine an area of contact thereby closing the gap such that the liquidis substantially removed between the deformable layer and thetransparent electrode in the area of contact. A plurality of switches isformed on the substrate for supplying control signals to the array ofpixels to selectively actuate the deformable members of the pixels,wherein each switch comprises an actuating member movable between anactive state and an inactive state, whereby in the active state anycontrol signal supplied to the switch passes through the switch, and inthe inactive state any control signal supplied to the switch isprevented from passing through the switch.

In alternate embodiments, the deformable member may include a reflectivesurface which contacts an insulation layer over the transparentelectrode in the second state. The dye may be black such that light isreflected from the area of contact when the deformable member is in thesecond state and light is absorbed in the gap when the deformable memberis in the first state. The dye may include Sudan Black or Naphthol BlueBlack. The deformable member may include a light absorbent surface whichcontacts the transparent electrode in the second state. The dye may bewhite such that light is reflected from the gap when the deformablemember is in the second state and light is absorbed in the area ofcontact when the deformable member is in the first state. The deformablemember is bistable having a hysteresis such that only the first andsecond states are permitted. Alternatively, the deformable member may beadjustable between a plurality of states thereby adjusting the gap toprovide reflected light on a grey scale (i.e., various intensities). Theswitches preferably include microelectromechanical switches and areformed simultaneously with the display elements.

In other embodiments, the deformable member is preferably actuated onhinges integrally formed with the deformable member. The device mayinclude an active area which may include a first seal region formaintaining the liquid in the active area. The display device mayfurther include a second seal region for maintaining an inert gastherein between the first seal region and the second seal region suchthat the plurality of switches exist in the inert gas. The transparentelectrode may form a data line for controlling the pixels and thedeformable member may form a gate line for controlling the pixels suchthat voltage differences provided by control signals between the gateline and the data line provide a force for actuating the deformablemember. The plurality of switches may be formed on the substrate forsupplying the control signals on gate lines and data lines to the arrayof pixels to selectively actuate the deformable members of the pixels. Ashift register may be included using a portion of the plurality ofswitches, the shift register is employed for addressing the gate lines.Switches may also be employed for demultiplexing the data lines toreduce the number of data driver chips and electrical connectionsneeded.

Another display device, in accordance with the invention includes asubstrate and an array of pixels formed on the substrate, each pixelincluding a transparent substrate and a deformable member electricallyactuated between a plurality of states. In each of the states, a liquidincluding a dye is disposed in a gap between the transparent substrateand the deformable member in an active area and wherein the gap isadjustable according to voltages applied to and stored by each pixelthereby reflecting light from the active area according to a grey scale(i.e., by varying the intensity of the reflected light).

In alternate embodiments, the deformable member may include a reflectivesurface for reflecting light through the transparent substrate andwherein the dye is black. The deformable member may include a lightabsorbent surface for absorbing light through the transparent substrateand wherein the dye is white. The deformable member is preferablyactuated on hinges integrally formed with the deformable member. Anactive device area may be included, and a first seal region may also beincluded for maintaining the liquid in the active area. The displaydevice may include a second seal region for maintaining an inert gastherein between the first seal region and the second seal region suchthat the plurality of switches exist in the inert gas. A shift registermay be included, and a portion of a plurality of switches may be used toconstruct the shift register. The shift register is preferably foraddressing gate lines which are used to activate switches in each pixelto connect the data lines to storage capacitors in each pixel. Switchesmay also be employed for demultiplexing the data lines to reduce thenumber of data driver chips and electrical connections needed. Datalines are independent of gate lines.

A method for fabricating a display device includes providing a topplate, patterning a transparent electrode in an active region on the topplate, forming an insulating layer on the transparent electrode,patterning a low reflectivity conductive material to form a sourceelectrode, a gate electrode and a drain electrode on the insulatinglayer outside the active area, patterning a sacrificial layer,patterning a metal layer to form deformable members having a gap betweenthe metal layer and the transparent electrode in the active area andswitches outside the active area such that upon activating the gateelectrode an electrical connection is made between the source electrodeand the drain electrode, removing the sacrificial layer and filling thegap with a liquid including a dye such that in a first state of thedeformable member the liquid is disposed in the gap between thetransparent electrode and the deformable member and wherein in a secondstate the deformable member contacts the insulating layer over thetransparent electrode to define an area of contact thereby closing thegap such that the liquid is substantially removed between the deformablelayer and the insulating layer over the transparent electrode in thearea of contact.

In alternate methods, the step of patterning the sacrificial layerpreferably includes the steps of forming a via hole through thesacrificial layer to the source electrode and forming a tip feature holeover the drain electrode such that upon patterning the metal layer acantilevered conductor is attached to the source electrode and includesa tip feature for contacting the drain electrode. The step of formingthe source electrode, the gate electrode, the drain electrode and ablack matrix concurrently from a low reflectivity conductive materialmay also be included.

Another method for fabricating a display device includes the steps ofpatterning a black matrix layer on a transparent substrate, depositing afirst insulation layer on the patterned black matrix layer, patterning atransparent conductor layer on the first insulation layer, depositing asecond insulation layer on the transparent conductor layer, depositing asacrificial layer on the second insulation layer for forming a gap of apredetermined distance between the second insulation layer over thetransparent conductor layer and deformable members, forming openings inthe sacrificial layer for providing support points for deformablemembers, patterning a metal layer to form deformable members andremoving the sacrificial layer to provide the gap.

In alternate methods, the step of filling the gap with a liquidincluding a dye such that in a first state of the deformable member theliquid is disposed in the gap between the transparent electrode and thedeformable member and wherein in a second state the deformable memberreduces the gap between the second insulation layer over the transparentelectrode and the deformable member such that the liquid issubstantially removed between the deformable member and the secondinsulation layer over the transparent electrode is preferably included.The sacrificial layer may include copper and the step of removing thesacrificial layer may include the step of removing the sacrificial layerby a wet etch process. The deformable members include deformablemirrors.

Yet another method for fabricating a deformable display device includesthe steps of patterning a transparent conductor layer in an active areaof a transparent substrate, forming an insulation layer over thetransparent conductor layer, patterning a conductive black matrix layeron the insulation layer outside the active area, the black matrix layerused for forming a drain electrode for switches, providing a sourceelectrode and a gate electrode for switches by patterning one of theblack matrix layer and the transparent conductor layer outside theactive area, patterning a sacrificial layer for forming features in thesacrificial layer for providing support points for the deformablemember's connections through the sacrificial layer (including tipfeatures for the switches) and patterning a metal layer on thesacrificial layer to form the deformable members and support points forthe deformable members, the deformable members including deformabledisplay members in the active area and switches outside the active area,removing the sacrificial layer to provide a predetermined gap betweenthe insulation layer over the transparent conductor and the deformabledisplay members and to provide cantilevered conductors for the switches,the cantilevered conductors attaching to the source electrode andincluding a tip feature for contacting the drain electrode when the gateelectrode is activated.

In other methods, the step of patterning a sacrificial layer may includethe steps of forming a via hole through the sacrificial layer to thesource electrode and forming a tip feature hole over the drain electrodesuch that upon patterning the metal layer the cantilevered conductor isattached to the source electrode and includes the tip feature forcontacting the drain electrode. The sacrificial layer may include aconductive top portion and a lower insulating portion and may furtherinclude the steps of forming dimples in the top portion and in a portionof the bottom portion for forming the cantilevered conductors forswitches and forming openings through the top and bottom portions toform vias through the sacrificial layer. The conductive top portion mayinclude copper and the lower insulating portion may include polyimide,the method may further include the steps of removing the top portionwith a wet etching process and removing the lower portion by a plasmaetching process. The deformable display members preferably includedeformable mirrors. The method may also include the step of filling thegap with a liquid including a dye such that in a first state of thedeformable display member the liquid is disposed in the gap between thetransparent electrode and the deformable display member and wherein in asecond state the deformable display member reduces the gap between theinsulation over the transparent electrode and the deformable displaymember such that the liquid is substantially removed between thedeformable display member and the transparent electrode.

These and other objects, features and advantages of the presentinvention will become apparent from the following detailed descriptionof illustrative embodiments thereof, which is to be read in connectionwith the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in detail in the following descriptionof preferred embodiments with reference to the following figureswherein:

FIG. 1 is a cross-sectional view of an embodiment in accordance with thepresent invention with a deformable mirror in a relaxed state;

FIG. 2 is a top plan view of the embodiment of FIG. 1 in accordance withthe present invention with a deformable mirror in a relaxed state;

FIG. 3 is a cross-sectional view of the embodiment of FIG. 1 inaccordance with the present invention with a deformable mirror in acollapsed state;

FIG. 4 is a top plan view of the embodiment of FIG. 1 in accordance withthe present invention with the deformable mirror in the collapsed state.

FIGS. 5 and 6 are a top plan view and cross-sectional view,respectively, showing a black matrix layer and an insulating layerdeposited on a top plate in accordance with the present invention;

FIGS. 7 is a top plan view showing a transparent electrode layer and anadditional insulating layer deposited on the insulating layer of FIG. 6in accordance with the present invention;

FIG. 8 is a cross-sectional view of a section taken along section line8—8 of FIG. 7.

FIGS. 9 and 10 are a top plan view and cross-sectional view,respectively, showing a sacrificial layer deposited and patterned on thedevice of FIG. 8 in accordance with the present invention;

FIGS. 11 and 12 are a top plan view and cross-sectional view,respectively, showing the sacrificial layer of FIGS. 9 and 10 removedand a metal layer used to form deformable mirrors and switches depositedin accordance with the present invention;

FIG. 13 is a top plan view of four pixels in accordance with the presentinvention;

FIG. 14 is a plot of mirror gap versus drive voltage in accordance withthe present invention;

FIGS. 15 and 16 are a top plan view and cross-sectional view,respectively, showing a conductor deposited in accordance with thepresent invention;

FIG. 17 is a cross-sectional view showing a sacrificial layer depositedand patterned on the device of FIG. 16 in accordance with the presentinvention;

FIGS. 18 and 19 are a top plan view and cross-sectional view,respectively, showing the sacrificial layer of FIG. 17 removed and ametal layer used to form deformable mirrors and switches deposited inaccordance with the present invention;

FIG. 20 is a schematic diagram of an assembled display in accordancewith the present invention;

FIG. 21 is a partial cross-sectional view taken along section line 21—21of FIG. 20 in accordance with the present invention;

FIG. 22 is a schematic diagram showing a shift register and datamultiplexing circuits constructed from MEM switches in accordance withthe present invention;

FIG. 23 is a top plan view of a display pixel having a black matrixmaterial deposited and patterned on a transparent substrate and coveredby a blanket insulation layer in accordance with the present invention;

FIG. 24 is a top plan view of the display pixel of FIG. 23 having a gatemetal deposited and patterned thereon in accordance with the presentinvention;

FIG. 25 is a top plan view of the display pixel of FIG. 24 having asacrificial layer deposited and patterned thereon, the sacrificial layershows dimples and hole for features to be formed in later processingsteps in accordance with the present invention;

FIG. 26 is a cross-sectional view of the display pixel of FIG. 25 takenat section line 26—26 of FIG. 25 in accordance the present invention;

FIG. 27 is a top plan view of the display pixel of FIG. 25 having a datametal/deformable member metal deposited and patterned thereon inaccordance with the present invention;

FIG. 28 is a cross-sectional view of the display pixel of FIG. 27 takenat section line 28—28 of FIG. 27 in accordance the present invention;

FIG. 29 is a top plan view of the display pixel of FIG. 27 showing aswitch and a deformable mirror formed after the sacrificial layeretching in accordance with the present invention;

FIG. 30 is a cross-sectional view of the display pixel of FIG. 29 takenat section line 30—30 of FIG. 29 in accordance the present invention;and

FIG. 31 is a cross-sectional view of the display pixel of FIG. 29 takenat section line 31—31 of FIG. 29 in accordance the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention includes an electrically actuated self supportingdeformable mirror which is operated in a liquid which includes a dye toform a reflective “paper-like” display. The deformable mirror is made bysurface micro-machining methods and has hysteresis so that passiveaddressing can be used. The deformable mirrors are formed on a topsubstrate of the display so that a gap is advantageously determined bythe thickness of a sacrificial layer.

The invention also includes the operation of the micro-machineddeformable mirrors in the fluid containing the dye. One advantage ofusing the dye is that a black and white display with high reflectivityand good contrast ratio is formed where the dye provides the black statewhen the deformable mirrors are in a relaxed state. When the mirrors arecollapsed against a transparent substrate (displacing the black dye), ahigh reflectivity metal such as Al or Ag provides a bright white state.A gap between the deformable mirror and top glass of about 2-3 micronsexists to permit flexure of the mirror.

Assuming a reflectivity of about 90% and an aperture ratio of about 80%,a brightness of about 72% is achievable.

Further, the fabrication process includes few masking steps which permitreduced manufacturing costs. With an additional masking step, theprocess is compatible with microelectromechanical (MEM) switches. Thefollowing describe MEM switches and are incorporated herein byreference: P. M. Zavracky, S. Majumder and N. E. McGruer,“Micromechanical Switches Fabricated using Ni surface micromachining”,Journal of Microelectromechanical systems, Vol. 6 No. 1 (1997) p.3; U.S.Pat. No. 4,674,180 to P. M. Zavracky et al.; and U.S. Pat. No. 5,638,946P. M. Zavracky. The MEM switches described in the above documents may beused for some of the addressing circuits such as a shift register foraddressing gate lines and data demultiplexing to reduce the number ofrequired contacts in these devices. This is desirable for aneconomically feasible high information content display where individualelectrical contacts for each gate and data line to the associated driverchips increase total display costs.

Referring to the figures in which like numerals represent the same orsimilar elements and initially to FIGS. 1 and 2, a display element 18 isshown in a relaxed state in accordance with the present invention. In apreferred embodiment a gate line 12 is provided using deformable mirrors33 which may be electrically interconnected to form gate lines 12.Further, a combination of voltages may be applied to data lines 14,which may formed by a continuous stripe of a transparent conductiveelectrode 30, such as an indium-tin oxide electrode. Deformable mirror33 may include at least two positions. One position is a relaxedposition (FIG. 1) and another is a collapsed position (FIG. 3). In oneembodiment, the relaxed position corresponds to an applied voltage lessthan a voltage (threshold voltage) needed to pull deformable mirror 33down , i.e. to collapse it. With deformable mirror 33 in the relaxedposition, a gap 35 between mirror, 33, and an insulator layer 32 isoccupied by a black dye 34.

Black dye 34 has a high optical extinction coefficient, is a goodelectrical insulator, has a high dielectric constant to lower thethreshold voltage for the mirrors and has a low viscosity. Black dye 34may include a single or combination of disazo dyes dissolved in asolvent such as EGME (2-methoxyethanol), acetone, toluene, etc. Apreferred dye may be Sudan Black B (also known as flat black BB orSolvent Black 3). Another possible disazo dye is Naphthol Blue Black(also known as Acid Black 1 or Amido Black 10B).

Using Sudan Black B, with a gap thickness of about 3 um, the reflectedlight in the dark state would be about 1.6%. For Naphthol Blue Black,with a gap thickness of about 2 um, the reflected light in the darkstate would be about 1.2%. If the reflectivity of the mirror is 80%, thecontrast ratio of the active area of the pixels would range from about50 to about 67. Note that the display operation does not require thatabsolutely all of the dye be removed from between deformable mirror 33and insulator layer 32 when the mirror is in the collapsed state. UsingSudan Black B, if a gap of, for example, 100 nm remains in the collapsedstate, the reflected light would be about 87% of the brightness ofreflected light if the gap was completely empty of dye. This woulddegrade the brightness and contrast ratio slightly but could beacceptable for many applications.

As shown in FIG. 2, black dye 34 absorbs light which is transmittedthrough a top glass 10, a transparent insulator 39 and transparentelectrode 30. Any remaining light is reflected by deformable mirror 33back through black dye 34. This results in little or no light beingreflected back to the viewer through the top plate 10. Black dye 34 maybe disposed in a reservoir 22 between deformable mirror 33 and asubstrate 20. A black matrix layer 31 is also included to absorb lightin regions surrounding transparent electrode 30.

Referring to FIGS. 3 and 4, element 18 is shown in a collapsed statewhere the combination of the voltage applied to gate line 12 and to dataline 14 is above the threshold voltage necessary to collapse mirror 33.When mirror 33 is collapsed, a mirror portion 27 moves toward insulator32 on integrally formed hinges 26, black dye 34 is displaced from gap 35and any light transmitted through top plate 10, transparent electrode30, and insulator layers 32 and 39 is reflected by mirror 33. As shownin FIG. 4, a pixel (mirror 33) appears bright when viewed through topplate 10. If gaps 35 and reservoirs 22 do not provide adequate area forrapid transport of black dye 34, i.e. dye displacement, additionalopenings can be made in the deformable mirrors. The displaced dye movesinto reservoir 22 between deformable mirror 33 and substrate 20.Reservoir 22 provides a region for which dye is displaced into asindividual deformable mirrors are collapsed or from which dye isprovided as individual deformable mirrors return to the relaxedposition.

Referring to FIGS. 5-12, the processing steps for fabricating thedeformable mirrors of the present invention wherein MEM switches are notincluded is shown. Referring to FIGS. 5 and 6, a black matrix layer 31is deposited on top plate 10. Top plate 10 may include a glass such assilicon based glass. Black matrix layer 31 preferably includes a lowreflectivity material, preferably formed of chromium oxide (Cr_(x)O_(y))and/or chromium (Cr), to reduce the reflection of light from the areasoutside of an active pixel region. Black matrix layer 31 may beconductive. Black matrix layer 31 may be patterned by standardlithography and wet etching techniques and overcoated with a conformallydeposited transparent insulator layer 39 such as silicon oxide (SiO₂) orSilicon Nitride (SiN_(x)) . As shown in FIGS. 7 and 8, transparentelectrode 30 is formed by depositing a transparent conductive layer,preferably indium-tin oxide (ITO), or another transparent conductor, toform data lines 14. Transparent electrode 30 is patterned by lithographyand wet or dry etching and overcoated with an insulator 32 which is alsotransparent.

As shown in FIGS. 9 and 10, the deposition of a sacrificial layer 36with a thickness equal to the desired thickness of gap 35 (FIG. 1) isperformed. Sacrificial layer 36 may include a material such as sputteredcopper (Cu), or Cu plated into a conducting seed layer. Sacrificiallayer 36 may be textured on its surface which contacts mirror 33 toprovide diffuse reflectivity for the mirror when formed in thesubsequent steps. The thickness of sacrificial layer 36 determines thedistance of the gap, 35, between deformable mirror 33 and insulatorlayer 32 on top plate 10 when the mirror is in the relaxed position (SeeFIG. 1). Sacrificial layer 36 is patterned as shown in FIG. 9 by coatingthe device with a photoresist, exposing and developing the resist, andusing a suitable wet etch. A mixture of phosphoric acid, acetic acid,nitric acid and water in the ratios of about 80%/5%/5%/10%,respectively, for example, may be used to perform the wet etch.Insulator layer 32 adjacent to sacrificial layer 36 is preferablypatterned at the same time as sacrificial layer 36 using wet or dryetching after which the photoresist is removed. In this example,transparent electrode 30 is used as an etch stop so that only insulator32 is patterned, and not insulator layer 39. Also, this ensuresdeformable mirror 33 is not electrically connected to black matrix layer31 as will be apparent from FIG. 12.

Referring to FIGS. 11 and 12, a photoresist layer is deposited andpatterned over sacrificial layer 36. Deformable mirrors 33 are formed byplating in exposed conductor pattern areas created by the photoresist toform a metal layer 37. The photoresist is spun on and patterned todefine the mask for plating mirrors 33, hinges 26, and other areas wherethe metal used for the deformable mirrors is desired. The patternedphotoresist is baked at about 150° C. to improve its chemical resistanceduring the plating step(s). Preferably, a preclean is performed in about10% aqueous hydrochloric acid prior to plating. The metal layer mayinclude nickel (Ni) deposited to the desired thickness by electroplatingfrom a commercial electroplating solution containing Ni. One alternativeis to plate an initial layer of silver (Ag) prior to the Ni plating.Other metals are contemplated for the metal layer, for example aluminum(Al). After the metal layer is plated, the photoresist layer is strippedand a selective wet etch is used to remove sacrificial layer 36 but notetch the metal layer. It is to be understood that the present inventiondoes not employ timed etching processes. Overetching will not damage thestructure or render it nonfunctional as described above in the priorart.

If copper (Cu) is used for sacrificial layer 36 and Ni for the metallayer, sacrificial layer 36 may be etched in a mixture of approximately40 parts water, 1 part hydrogen peroxide, and 8 parts ammonium hydroxidewithout damage to the Ni, for example. If a Ag layer has not alreadybeen added and higher reflectivity mirror is desired, the Ni metal layercan be electroplated with Ag after sacrificial layer 36 is removed. Thepattern in which the metal layer is plated produces deformable mirror 33with hinges 26 on at least two sides.

Referring to FIG. 13, deformable mirrors 33 for each pixel areelectrically interconnected to the adjoining deformable mirrors 33 inthe direction of arrow “A” through hinges 26 to form individual gatelines 12. Deformable mirrors 33 are electrically isolated fromtransparent electrodes 30 (FIG. 12). Segments of transparent electrodes30 in each pixel are electrically interconnected to the adjoiningsegments of transparent electrodes 30 in the vertical direction to formthe individual data lines 14. Thus, an array of pixels 50 is formed ontop plate 10. Array of pixels 50 is electrically connected by gate lines12 and data lines 14 to which driver chips can be attached at the edges(at the end of the array). Data lines 14 and mirror metal gate lines 12are extended beyond the active display area and past a glue seal regionat the periphery of the array to electrically connect with bond pads ina tab area (formed either of ITO, gate metal, black matrix material, ora combination of these conductive layers) where driver chips cansubsequently be attached by the use of anisotropic conductive film (ACF)or other techniques.

Referring to FIG. 14, a display device having array of pixels 50 isaddressed according to the present invention by sequentially selectingeach of the gate lines and using the data lines to address each of thepixels on the selected gate line. The needed addressing voltages canbest be understood with reference to FIG. 14 which shows a schematic ofmirror gap 35 versus the data to gate voltage. In a preferredembodiment, the threshold voltage for collapse of the mirror “Vm(cp)” isabout 19V and the threshold voltage for release “Vm(rl)” of a collapsedmirror is about 1 V. The actual magnitude of the threshold voltage forcollapsing the deformable mirror and the mirror gap value isillustratively shown in FIG. 14. The selected gate line is held at“Vg(on)” and the gate lines not being addressed are held at “Vg(hold)”.The data voltages are “Vd(on)” for a white pixel (collapse position ofdeformable mirror) and “Vd(off)” for a black pixel (relaxed position ofdeformable mirror). The combination of the Vg(on) and Vg(hold) and theVd(on) and Vd(off) are selected so that:

Vg(on)+Vd(on)>Vm(cp)

Vg(on)+Vd(off)<Vm(cp)

Vm(rl)<Vg(hold)+Vd(on or off)<Vm(cp)

Appropriate drive voltage values for the above case may be, for example:Vg(on)=15V, Vg(hold)=5V, Vd(on)=10V, and Vd(off)=0V. Prior to selectinga line and writing the data to it, it is necessary to release anycollapsed mirrors. This may be accomplished by applying “Vg(clear)” tothe next line to be addressed just prior to selecting it where:

Vg(clear)+Vd(on or off)<Vm(rl)

or, alternatively, when selecting a line, prior to application ofVg(on), Vg(clear) can be applied while setting the data voltages toVd(clear) such that:

Vg(clear)+Vd(clear)<Vm(rl)

Since display elements 18 are bistable (i.e. have hysteresis as shown inFIG. 14), advantageously, there is no degradation of contrast ratio asthe number of lines is increased as is found for passive matrix liquidcrystal displays. In other embodiments, mirror gap is variedproportionally with the data-gate voltage to provide a non-hysteresismode wherein light may be reflected according to a grey scale (varyingintensities of light), i.e., proportionally with the gap.

Referring now to FIGS. 15-19, the processing steps to fabricate analternate embodiment of the present invention are described.Microelectromechanical (MEM) switches are provided outside an array ofpixels to reduce the number of driver chips and electrical contactsneeded. FIGS. 15-19 show the fabrication of an MEM switch only. Theprocessing for the pixels is the same as shown and described aboveexcept that transparent electrode 30 is patterned first, an insulator 32is deposited, and when black matrix layer 31 is patterned, black matrixlayer 31 is segmented and used to provide redundancy for the gate lines.This change ensures that there is no insulator over black matrix layer31 so that black matrix layer 31 can be used for a drain contact pad.Also, a lithography step is used to define a tip feature region of theMEM switches. Contacts to the data lines are formed using the mirrormetal. The MEM switches are fabricated concurrently with the deformablemirrors in accordance with the present invention.

Referring to FIGS. 15 and 16, the fabrication of an MEM switch 100begins by patterning a conductive layer 131 to form a source 104, gate106, and drain 108 of the switch on an insulator 132 and a top plate110. Source 104, gate 106 and drain 108 may be patterned from differentlayers. Drain 108 is preferably patterned from the black matrix butsource 104 and gate 106 may be patterned either from the ITO or theconductive black matrix layer. If ITO is used for source 104, theinsulator layer over source 104 is removed during processing so thatdeformable mirror 133 makes electrical contact with source 104. Theblack matrix material or transparent electrode material for source 104,gate 106, and drain 108 is preferably deposited concurrently with thesimilar materials included for the active areas, i.e., for processingthe deformable mirrors.

As shown in FIG. 17, a sacrificial layer 136 is deposited. Twopatterning steps are preferably used to first open a switch tip feature102 preferably to a depth of slightly greater than about ⅔ ofsacrificial layer 136 thickness, and to second open sacrificial layer 36fully down to black matrix layer 131 to form a source contact hole 103.The depth of switch tip feature 102 is adequate to ensure that switch100 operates in a non-hysteresis mode.

Referring to FIGS. 18 and 19, a metal is deposited as described above toform an actuating member 133. A voltage applied between gate 106 andsource 104 (to which actuating member 133 is cantilevered from) of theMEM switch which exceeds the threshold voltage actuates switch 100.Switch 100 closure shorts tip feature 101 of actuating member 133 todrain 108, thereby electrically connecting source 104 and drain 108.When the applied voltage is reduced below the threshold voltage, switch100 opens up and source 104 and drain 108 are again electricallyisolated. Further details on these processing steps and the operation ofMEM shunts can be found in Zavracky et al.

An assembled display is shown schematically in FIGS. 20 and 21 for theembodiment with integrated MEM switches. A top plate 10 has deformablemirrors in a display or active area 141. Top plate 10 includes MEMswitches 100 along one or more edges to form a shift register to addressthe gate lines. Also included are additional MEM switches to demultiplexdata signals on the data lines. Top plate 10 is attached to substrate 20using a glue seal region formed from materials such as epoxy. In thecase of integrated MEM switches, two separate regions may be formedbetween top plate 10 and bottom plate 20. One region includes a dye glueseal 142 which includes deformable mirrors 133 to be used in conjunctionwith dye and another region including the MEM switches includes drynitrogen or another inert gas and is sealed by a gas glue seal 143. Thisis advantageous as the switches operate faster in gas than liquid due tothe lower viscosity of gas. External to region 143 is a tab region 144where external drivers may be attached to metal bond pads or otherconnectors.

Referring to FIG. 22, one embodiment of a display 150 in accordance withthe present invention is shown. Display 150 includes an array of pixels154 including deformable mirrors 133. Integrated MEM switches 152 form ashift register 156 to address gate lines 12. Integrated MEM switches 152may also form circuits 157 to demultiplex data signals on data lines 14.This reduces the number of electrical contacts needed. Deformablemirrors 133 and switches 152 are formed concurrently during devicefabrication. The display devices and switches used are both fabricatedwith the same process steps and are both electrostatically actuated witha mechanical restoring force, but the switches are constrained by thetip feature to operate in a non-hysteresis mode whereas the displayelements are not constrained and hence are bistable.

Referring to FIGS. 23-31, in an alternative embodiment of a reflectivedisplay, an active matrix is used and the deformable member may beadjusted to a number of positions to vary the intensity of the reflectedlight. A top plan view and their respective cross-sections of a singlepixel are shown in FIGS. 23-31. The processing steps and structure aresimilar to those described above. In FIG. 23, a conductive black matrix(BM) 202 is deposited and patterned on a transparent substrate 200.Transparent substrate 200 includes a thin insulating layer 203 thereon(FIG. 26). Black matrix 202 is covered with a transparent insulatorlayer 204. In FIG. 24, a gate metal layer 206 is deposited and patternedby conventional methods such as plating Ni on a seed layer in patternedresist and subsequently removing the resist and seed layer to formportion of a storage capacitor 207 and gate lines 209. In FIGS. 25 and26, a sacrificial layer 208 is deposited and patterned. Sacrificiallayer 208 preferably includes two layers. A bottom portion 221 mayinclude ⅔ or more of the total thickness of sacrificial layer 208 whichis preferably polyimide (a transparent polymer), and a top portion 223may include ⅓ or less of the total thickness of sacrificial layer 208which is preferably copper. The tip regions 210 are formed by patterningthe copper layer and part of the 34polyimide layer so that the tip depthis about ⅔ of or greater than the total gap thickness. Via holes 212 arepatterned through both the copper, polyimide layer and insulationstopping at the gate metal or BM layers.

In FIGS. 27 and 28, conductive material for data lines 214 anddeformable beams or hinges 216 are patterned by plating in patternedresist, using a metal 215 such as Ni. Than sacrificial layer 208 isremoved by selective wet etching (top copper portion) and the polyimidelayer (bottom portion)is selectively removed by plasma etching except ina region under a deformable mirror 218 where the polyimide remains toform a spacer 220 as shown in FIGS. 29 and 30. The plasma etching ofpolyimide can be directional, depending on the process conditions used,and proceeds laterally under the deformable metal features at acontrolled rate. This allows the polyimide to be removed from undernarrow features such as switches 222, shown in FIG. 31, or bending beams(hinges) 216 but not from under large features such as deformable mirror218. As an alternative, sacrificial layer 208 may include a bottom ⅓ orless of copper and a top ⅔ or more of polyimide. In this case, polyimidespacers 220 is attached to the bottom of deformable mirror 218 and thepolyimide is etched first and the copper second where the tip featuresis only patterned in the polyimide layer.

Deformable mirror 218 is constrained by the polyimide to anon-hysteresis mode where the gap is controlled by the voltage stored onstorage capacitor 207. Storage capacitor 207 is formed between the gatemetal and the black matrix. Electrical contacts to the black matrix areformed outside the array region using the via pattern and the same metalas the data lines. As is usual for an active matrix device, the voltageon the storage capacitor is transferred from the data line when the gateline is selected and switch 222 is closed connecting the data line tothe storage capacitor. When the gate line is not selected, the switch isopen and the voltage is maintained by the storage capacitor. The voltagedifference between the storage capacitor and the previous (non-selected)gate line, to which bending beam 216 and deformable mirror 218 areconnected, controls the deflection of the bending beam and hence thedisplacement of the deformable mirror. The gap (gap 35 as shown inFIG. 1) between deformable mirror and the polyimide spacer determinesthe thickness of dye which incident light traverses before beingreflected from the mirror and hence the intensity of the reflectedlight.

Although described in terms of black dye and a reflective mirror, thepresent invention is applicable to other types of deformable mirrordisplays, for example, white dye and a black (non-reflective) mirror.Also, a subframe time modulation could be implemented to provide greyscale.

Having described preferred embodiments of a micromechanical displays andmethod for fabrication of same (which are intended to be illustrativeand not limiting), it is noted that modifications and variations can bemade by persons skilled in the art in light of the above teachings. Itis therefore to be understood that changes may be made in the particularembodiments of the invention disclosed which are within the scope andspirit of the invention as outlined by the appended claims. Having thusdescribed the invention with the details and particularity required bythe patent laws, what is claimed and desired protected by Letters Patentis set forth in the appended claims.

What is claimed is:
 1. A display device comprising: a transparentsubstrate; an array of pixels formed on the substrate, each pixelcomprising a transparent electrode and a deformable member electricallyactuated between a first state and a second state, wherein in the firststate a liquid including a dye is disposed in a gap between thetransparent electrode and the deformable member and wherein in thesecond state the defonnable member reduces the gap between thetransparent electrode and the deformable member such that the liquid issubstantially removed between the deformable layer and the transparentelectrode; and a plurality of switches formed on the substrate forsupplying control signals to the array of pixels to selectively actuatethe deformable members of the pixels, wherein each switch comprises anactuating member movable between an active state and an inactive state,whereby in the active state any control signal supplied to the switchpasses through the switch, and in the inactive state any control signalsupplied to the switch is prevented from passing through the switch. 2.The display device as recited in claim 1, wherein the deformable memberincludes a reflective surface which reduces the gap by displacing aportion of the liquid when the deformable member is in the second state.3. The display device as recited in claim 2, wherein the dye is blacksuch that light is reflected from the deformable member, when thedeformable member is in the second state and light is absorbed in thegap when the deformable member is in the first state.
 4. The displaydevice as recited in claim 3, wherein the dye includes Sudan Black. 5.The display device as recited in claim 3, wherein the dye includesNaphthol Blue Black.
 6. The display device as recited in claim 1,wherein the deformable member includes a light absorbent surface whichcontacts an insulation layer over the transparent electrode in thesecond state.
 7. The display device as recited in claim 6, wherein thedye is white such that light is reflected from the gap when thedeformable member is in the second state and light is absorbed on thedeformable member when the deformable member is in the first state. 8.The display device as recited in claim 1, wherein the deformable memberis bistable having a hysteresis such that only the first and secondstates are permitted.
 9. The display device as recited in claim 1,wherein the switches include microelectromechanical switches.
 10. Thedisplay device as recited in claim 1, wherein the deformable member isactuated on hinges integrally formed with the deformable member.
 11. Thedisplay device as recited in claim 1, further comprising an active areaincluding the liquid therein and a first seal region for maintaining theliquid in the active area.
 12. The display device as recited in claim11, further comprising a second seal region for maintaining an inert gastherein between the first seal region and the second seal region suchthat the plurality of switches exist in the inert gas.
 13. The displaydevice as recited in claim 11, wherein the transparent electrode forms adata line for controlling the pixels and the deformable member forms agate line for controlling the pixels such that voltage differencesprovided by control signals between the gate line and the data lineprovide a force for actuating the deformable member; and the pluralityof switches are formed on the substrate for supplying the controlsignals on gate lines and data lines to the array of pixels toselectively actuate the deformable members of the pixels.
 14. Thedisplay device as recited in claim 13, further comprising a shiftregister for addressing the gate lines, the shift register including aportion of the plurality of switches and another portion of theplurality of switches for demultiplexing the data lines.
 15. A displaydevice comprising: a substrate; an array of pixels formed on thesubstrate, each pixel comprising an electrically actuated switch, astorage capacitor, a transparent substrate and a deformable memberelectrically actuated between a plurality of states, wherein in each ofthe states a liquid including a dye is disposed in a gap between atransparent substrate and the deformable member in an active area andwherein the gap is adjustable according to voltages applied to thedeformable member and the storage capacitor to change an amount ofliquid disposed between the transparent substrate and a deformablemember thereby reflecting light from the active area according todifferent intensities.
 16. The display device as recited in claim 15,wherein the deformable member includes a reflective surface forreflecting light through the transparent substrate and wherein the dyeis black.
 17. The display device as recited in claim 15, wherein thedeformable member includes a light absorbent surface for absorbing lightthrough the transparent substrate and wherein the dye is white.
 18. Thedisplay device as recited in claim 15, wherein the deformable member isactuated on hinges integrally formed with the deformable member.
 19. Thedisplay device as recited in claim 15, further comprising an activedevice area including the liquid therein and a first seal region formaintaining the liquid in the active area.
 20. The display device asrecited in claim 19, further comprising a second seal region formaintaining an inert gas therein between the first seal region and thesecond seal region such that the plurality of switches exist in theinert gas.
 21. The display device as recited in claim 15, furthercomprising a shift register for addressing gate lines which are coupledto each pixel.
 22. The display device as recited in claim 15, whereinthe switches include microelectromechanical switches.